The proposed modification would create a separate subbasin that comprises the portion of the Tracy Subbasin that lies within Contra Costa County. The current subbasin includes areas of San Joaquin County and Contra Costa County.
The Contra Costa County Line along Old River would be the new eastern boundary of the new subbasin and the Contra Costa-Alameda County Line would be the new southern boundary.
The new subbasin would be bounded by the Coast Range to the west (unchanged), Old River to the east (new), San Joaquin River to the north (unchanged), and Alameda County Line to the south (new). The proposed boundary modifications are consistent in nature with the existing Bulletin 118 boundaries: Old River replaces the San Joaquin River (river) and Alameda County Line replaces the Stanislaus County Line (geopolitical) to form the new subbasin within Contra Costa County.